The present invention relates to an angular position sensor or transducer located between two coaxial parts, which are either stationary or in relative rotation and having a magnetic reluctance which varies as a function of said angular position. It also relates to an angular position determination means equipped with several of these sensors or transducers.
There is a particular need for very accurate angular position measuring equipment in robotics, where it is necessary to accurately determine the working angle of the joints of the arms of robots. The checking of the very fine angular displacements can be carried out as a result of a series of devices which, coupled to one another or extended by angle multipliers (particularly gears) make it possible to discretize the circular movements in a very precise manner.
The process based on magnetic reluctance variation consists of passing an alternating electric signal into a coil surrounding a ferromagnetic structure. Thus, a magnetic flux is produced, which flows in a magnetic circuit formed from two almost contiguous parts which effect a relative displacement. The alternating signal passed into the electric circuit is closely dependent on said magnetic flux and can consequently characterise the same. As a function of their characteristics, the sensors make it possible to obtain the relative position of the two parts forming the magnetic circuit.
The equipment exploiting this phenomenon and applying it to the determination of the angular position comprise a rotor and a stator having teeth separated by an air gap. The notion of stator and rotor is not very significant for this application of angular position sensors, because these two concepts can also designate the inductor and the armature, the internal part as well as the external part, or even two parts arranged in parallel on either side of a planer air gap. This remark also applies to the present invention.
The hitherto developed angular position sensors able to supply very accurate measurements are of two types, examples of which are shown in FIGS. 1 and 2. According to the first of these examples, the part not carrying coils is provided with M patterns (studs or teeth) regularly distributed over its circumference, whereas the inducting part carries n.times.M smaller patterns, whereof each group of n patterns is surrounded by a coil supplied by an alternating current, whose phase differs by 2.pi./n from that supplying adjacent coils, with the exception of the two-phase coil for which 4M patterns are necessary.
Thus, the sensor comprises repeat patterns displaced by 2.pi./M radians. The electric angle is said to be equal to the mechanical angle multiplied by M. If M is large, the measurements are in principle more precise, but there are soon limitations due to uncertainties concerning the machining of parts, particularly if polyphase currents are supplied which require more numerous patterns on the part equipped with the coils.
A second type of sensor or transducer shown in FIG. 2 and taken from the article "A New High Accuracy Angular Position Transducer", PCIM, September 1985, p. 55 by I. B. Cushing shows M racks on the rotor or stator, each equipped with an almost equal number of teeth. The coils, which in this case are located on the stator, are supplied by polyphase alternating currents. The major feature of this sensor is that the racks of one of the parts are not regularly spaced, so that the teeth of the racks carrying the coils of the same phase are always arranged in a similar manner facing the teeth of the other part. Moreover, a rotation of the rotor which brings a rack of the stator in front of the teeth of the rotor, in the same way as another rack of the stator into the starting position, corresponds to an electric angle of alpha alpha, being the phase displacement between the currents supplying the coils of these two stator racks.
Thus, the principle is the same as in the previous case and hereagain manufacturing tolerances impose practical limitations to the number of patterns which can be used.
The present invention makes it possible to obviate this limitation by means of an angular position sensor of simple manufacture and constituted more particularly by a rotor and a stator having the same number of regularly spaced teeth. According to a preferred embodiment of the invention, several of these sensors are associated in parallel and form an angular position determination means making it possible to obtain a finer angular step.